This invention relates to an electronic ignition system for a spark-ignition internal combustion engine and, more particularly, relates to a solid state ignition system capable of providing substantially constant dwell time, as distinguished from dwell angle, over a range of engine speeds. At speeds in excess of this range, the ignition system provides a dwell time which gradually decreases but which remains very satisfactory for most motor vehicle applications of spark-ignition internal combustion engines. The ignition system of the invention also provides circuitry for limiting the current in the ignition coil primary winding to a predetermined maximum level.
Recently, it has become desirable to provide an electronic ignition system for spark-ignition internal combustion engines wherein the energy supplied to the spark plugs on the secondary side of the ignition coil is at a higher level than in conventional ignition systems. Moreover, it has become desirable to provide a higher ignition coil secondary voltage level. In conventional ignition systems, which utilize breaker points to control current in the ignition coil primary winding, and in most presently available electronic ignition systems, this is either impractical or impossible.
To facilitate an understanding of the present invention, it is desirable to define certain terms used herein. Thus, for the purposes of the present invention, the term "ignition cycle" refers to the time period between consecutive firings or spark discharges of an internal combustion engine ignition system. The term 'dwell time" refers to the time, expressed in units of time, during which a current, other than leakage current if any, is present in the ignition coil primary winding. The term 'dwell angle" is expressed in angular units and represents the fraction or angular portion of the ignition cycle represented by the dwell time. Thus, an ignition system which provides a constant dwell time has a dwell angle which varies with engine speed, and conversely, an ignition system which provides a constant dwell angle over a range of engine speeds has a variable dwell time over such range of engine speeds.
Conventional ignition systems typically provide a constant dwell angle. This results from the fact that such systems utilize breaker points controlled by a cam driven by the engine distributor. The cam causes the breaker points, which are connected in series with the ignition coil primary winding, to remain open for a predetermined substantially constant angle of rotation of the distributor shaft. Such a conventional system has a variable dwell time because the breaker points are closed for a considerable length of time at low engine speeds and, as engine increases, the length of time the points remain closed decreases due to the greater angular velocity of the distributor shaft. Most of the previously proposed inductive electronic ignition systems for internal combustion engines have provided constant dwell angle.
In an inductive ignition system, it is necessary to establish a predetermined current in the ignition coil primary winding to insure adequate sparking potential on the secondary of the ignition coil. Since this current is established by turning on a switch that permits electrical charge to flow through the ignition coil primary winding, the length of time required to establish the predetermined necessary current is determined by the resistance-inductance circuit of the ignition coil primary winding. Once an electrical potential is applied to the ignition coil primary winding, the current exponentially builds up, in the manner for an inductive circuit, to a certain level and, with respect to the generation of a high potential on the secondary of the ignition coil, little is gained by permitting this current to continue for a greater length of time. Moreover, continuance of this current once it has reached a certain level results in a significant waste of power of the source of electrical energy. Also, it can cause overheating of the ignition coil or, were higher ignition coil secondary voltages and energy levels to be provided, require an unduly large ignition coil and other ignition system components. Thus, it is desirable to provide a constant dwell time in an ignition system, a dwell time sufficient only to permit the ignition system primary current to achieve a predetermined satisfactory level sufficient to produce an adequate sparking potential and to produce such sparking potential as soon as practicable after this predetermined current level has been achieved in the ignition coil primary winding. An ignition system capable of providing this type of operation is described in U.S. Pat. No. 3,605,713 issued Sept. 20, 1971, to P. D. Le Masters et al. The present invention provides a similar result, but achieves that result in a substantially different manner than that described in U.S. Pat. No. 3,605,713.